Electrically and thermally enhanced integrated-circuit package

ABSTRACT

An encapsulated electrically and thermally enhanced integrated circuit is disclosed. An integrated-circuit die is attached to a thermally conductive, electrically-insulated substrate. A lead frame having inwardly-extending bonding fingers has the bottom sides thereof attached to the top of the substrate. A contiguous layer of insulating material is bonded to the top sides of the bonding fingers, such that the layer of insulating material peripherally surrounds the integrated-circuit die. A conductive layer of material is then bonded to the top of the insulating layer. A second layer of insulating material followed by a second conductive layer may be bonded on top of the first conductive layer. Electrical connections are made from the integrated-circuit die to the conductive layers surrounding the die. The device is then encapsulated in a plastic material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuit manufacturingtechniques and particularly to plastic encapsulation techniques for ofelectrically and thermally enhanced integrated-circuit package designs.

2. Prior Art

Conventional integrated circuit devices are commonly manufactured byattaching an integrated-circuit die to a lead frame. The lead frameincludes a centrally located die-attach paddle, upon which theintegrated-circuit die is mounted. The lead frame also includes aplurality of conductive leads, the inner ends of which converge at thedie-attach paddle. Bonding finger portions at the inner ends of theleads are used to make electrical connections between theintegrated-circuit die and the conductive leads of the lead frame. Theentire assembly described above is encapsulated in a molded plasticmaterial to form a molded-plastic body.

As semiconductor technologies improve, individual semiconductor circuitdevices become smaller, allowing for greater die packing densities andincreased operating speeds. However, these advancements present newproblems as well. For example, as operating speeds increase, theinductance of the leads of the lead frame causes signal cross-talk andground bounce. Additionally, as die densities increase, additional heatis produced by the die which can result in decreased operatingperformance or even failure of the integrated-circuit die.

As the operating speeds of the circuits on an integrated-circuit dieincrease, the inductance problems become more significant. Inductancecan cause the VSS voltage on a ground bus to vary as various deviceswithin the integrated circuit are switched on and off. Inductance in theleads of the integrated circuit and the lead frame can cause the VSSvoltage to vary on the ground bus. This variation is referred to asground bounce and degrades the high speed performance of the integratedcircuit.

One technique for reducing inductance is to designate a number of leadsor conductors which are connected in parallel. This increases the numberof I/O package pins being required.

Another technique for reducing inductance is to use a multilayerprinted-circuit substrate. This type of package is extremely expensivedue to high manufacturing costs and low production yields for theprinted circuit substrate.

A number of techniques have been used to alleviate the problems causedby heat buildup in a densely packed integrated-circuit die. Onetechnique uses a thermally-conductive, electrically-insulated substrateto which the integrated-circuit die is attached. The bottom surface ofthe die is attached to the top surface of the thermally conductivesubstrate so that the thermally conductive substrate can transfer heataway from the integrated-circuit die. The thermally-conductive,electrically-insulated substrate is formed, for example, of a materialsuch as alumina nitride. For good thermal operation, the thermallyconductive substrates are relatively thick, significantly thicker thanthe die-attach paddle of a conventional lead frame.

However, because of this increased thickness and bulk of the substrate,the thicker thermally conductive substrates can impede or restrict theflow of the plastic molding compound to certain areas of the mold duringencapsulation of the integrated circuit. The plastic molding materialflows more freely over the top of the integrated circuit than under theintegrated circuit. As a result of differences in flow rates, the airwhich is forced through the molds by the molding compound may be trappedon the bottom side of a package, which leaves blowholes or voids in thebody of the molded package. Consequently, use of the thicker thermallyconductive substrates has created moldability problems, such asformation of voids.

Consequently, the need exist for an integrated-circuit packageconfiguration which has improved thermal performance, reducedinductance, and superior moldability characteristics.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an integratedcircuit package which has improved thermal characteristics and reducedelectrical inductance in a molded plastic package free of voids.

This object is achieved with an electrically and thermally enhancedintegrated circuit which reduces inductance and ground noise problems,and which provides a balanced flow of plastic molding material aroundthe integrated circuit. This is accomplished by peripherally surroundingan integrated-circuit die with one or more layers of conductive materialseparated from each other by layers of insulating material, and usingthe conductive layers as VSS ground and VDD power planes. During moldingof a package body, the added conductive layer also serves as a barrierto provide for balanced flow of a molding compound around the integratedcircuit.

An encapsulated, molded-plastic package for an electrically andthermally-enhanced, integrated circuit is provided. Anintegrated-circuit die is attached to the central area of the topsurface of a thermally conductive, electrically-insulated substrate. Alead frame with inwardly-extending bonding fingers is also attached tothe thermally conductive, electrically-insulated substrate. The bottomsides of the bonding fingers are attached to the top surface of thesubstrate such that the bonding fingers of the lead frame peripherallysurround the integrated-circuit die.

A layer of insulating material is then bonded to the top side of theinwardly-extending bonding fingers, such that the bonding fingers aredisposed between the layer of insulating material and the top of thethermally conductive, electrically-insulated substrate. The insulatinglayer is applied such that the insulating material peripherallysurrounds or "rings" the integrated-circuit die. A layer of conductivematerial is then bonded directly on top of the insulating material, suchthat the layer of insulating material is disposed between the conductivelayer and the bonding fingers. The conductive layer of material is alsoapplied in a contiguous layer such that it peripherally surrounds or"rings" the integrated-circuit die. Electrical connections are then madefrom the integrated-circuit die to the conductive layer surrounding thedie. After the electrical connections have been made, the entire deviceis encapsulated in a plastic material.

In one embodiment of the invention, additional alternating layers ofinsulating and conductive material are formed on top of the first layersof insulating and conductive material. In such arrangements, one of theconductive layers is used a VDD power plane, and the other conductivelayer is used as a VSS ground plane. In so doing, ground and powerterminals on the integrated-circuit die can be attached directly to theground or power conductive planes. Additionally, the alternating layersof insulating and conductive material provide resistance to the flow ofthe molding compound over the top of the integrated-circuit die. Therestriction caused by the layers of material slows the flow of themolding material over the top of the die just as the thermallyconductive substrate slows the flow of the molding compound under theintegrated-circuit die. As a result, a balanced flow of the moldingcompound is achieved in the mold cavity and a plastic package free ofblowholes or voids can be formed.

In another embodiment of the present invention, the thermallyconductive, electrically insulated substrate is a copper slug which isseparated from the integrated-circuit die and the bottom side of thebonding fingers of the lead frame by a layer of electrically insulatingmaterial. The copper slug is of a size such that when the entireassembly is encapsulated in plastic, the bottom portion of the slugremains exposed. That is, the bottom of the copper slug is notencapsulated in plastic, and thus the exposed copper surface allows foreffective dissipation of heat away from the integrated-circuit die.

A method is also provided for producing an electrically and thermallyenhanced integrated circuit. The method includes the steps of attachingan integrated-circuit die to a thermally-conductive, electricallyinsulated substrate, attaching bonding fingers of a lead frame to thetop surface of the substrate, and bonding alternating layers ofinsulating and conductive material around the integrated-circuit diesuch that the layers of material peripherally or surround theintegrated-circuit die. The layers of conductive material are then usedas ground or power planes. Electrical ground and or power connectionsare then made from the integrated-circuit circuit die to the conductivelayers, and the integrated circuit is encapsulated in a plasticmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

FIG. 1 is a partially cutaway plan view of an electrically and thermallyenhanced integrated circuit package assembly, according to the presentinvention.

FIG. 2 is a perspective, exploded view of the assembly of FIG. 1.

FIG. 3 is a sectional, elevation view of the assembly of FIG. 1,including electrical wire-bonds.

FIG. 4 is a sectional view of an upper mold half and a lower mold halfcontaining a thermally conductive, electrically-insulated substrateassembly having conductive layers formed on the top side of theintegrated-circuit die which balances the flow of plastic moldingmaterial in the upper and lower halves of the mold and tends toeliminate formation of voids in the molded plastic body of the package.

FIG. 5 is a sectional, elevation view of an alternative embodiment of anintegrated-circuit assembly which uses an insulated copper slug toprovide enhanced thermal characteristics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 and FIG. 2 of the drawings, an intermediate packageassembly 8 is shown according to the invention. FIG. 1 shows a plan viewof the intermediate package assembly 8 according to the presentinvention.FIG. 2 shows the same assembly 8 in an exploded, perspectiveview. This assembly 8 is typically encapsulated into a final packageassembly by molding a body formed of molded plastic material around thisassembly 8.

The intermediate assembly 8 includes an integrated-circuit die 10 havingwire-bonding pads (typically shown as 12) formed on the top surfacethereof. The integrated-circuit die 10 is attached to the center area ofathermally conductive substrate 14. The thermally conductive substrate14 can be formed from any one of many materials well known in the art,such as aluminum nitride or silicon carbide (SC) material.Alternatively, a slug of copper or similar conductive material, and asuitable insulator isused for the thermally-conductive,electrically-insulated substrate 14.

A lead frame 16 has a number of inwardly-extending leads 18 with theinner ends of the leads 18 forming bonding fingers 20. The bottom of theleads 18 and the bonding fingers 20 are attached to the top side of thethermally-conductive, electrically-insulating substrate 14. The leadfingers are attached to the substrate using a one mil thick film ofR-Flex1000 film material. The lead frame 16 with its inwardly-extendingbonding fingers 20 at the inner ends of the leads 18 are typicallyformed of a metal such as copper or alloy 42. The bonding fingers 20 ofthe lead frame16 extend around the outer margins of the top surface ofthe substrate 14, as illustrated.

A first insulating layer 26 is formed, for example, of one-mil R-Flex1000 film material and is positioned over the leads 18 of the lead framealong the outer margins of the top surface of the thermally conductive,electrically-insulated substrate 14. The bottom surface of a firstconductive layer 28 is positioned over and bonded to the insulatinglayer 26.

FIG. 1 illustrates that the first conductive layer 28 and the firstinsulating layer 26 are each formed as open rectangular window-framemembers having widths which extend from the outer edges of the substrate14 to near the bonding fingers 20.

A second insulating layer 30 is positioned over and is bonded to the topsurface of the first conductive layer 28 along its outer margins. Asecondconductive layer 32 is positioned over and bonded to the secondinsulating layer 30. The second insulating layer 30 and the secondconductive layer are also formed as open window-frame members withwidths less than the widths of the first conductive layer 28 and thefirst insulating layer 26.This difference in widths exposes the topsurface of the second conductive layer 32 and a substantial portion ofthe first conductive layer 28. A staircase structure is formed leaving aportion of the first conductive layer 28 and the second conductive layer32 uncovered and available for electrical connections. The first andsecond layers 28, 32 of conductive material are formed of a materialsuch as gold-plated copper or of other suitable conductive materials.

The assembly 8 is encapsulated in a molded package body formed of aplasticmaterial, which is molded around the assembly 8 to form a finalintegrated-circuit package assembly.

FIG. 3 shows a sectional, elevation view of the assembly of FIGS. 1 and2. A number of bonding wires are illustrated as connecting therespective bonding pads on the integrated-circuit die 10 to variouspoints on the package assembly. The first and the second conductivelayers 28 and 32 areused as large-area, low-inductance conductive planesfor appropriate supplyvoltages. The conductive layer 28 is used, forexample, as a VSS ground plane and the conductive layer 32 is used as aVDD power plane.

The top surface of the integrated-circuit die 10 has a number of bondingpads formed thereon, some of those bonding pads are used for power andground connections and some are used for signal input/output (I/O)connections. A typical bonding pad providing power to the die 10 isconnected by a wire-bonded wire 34 directly to the VSS power plane 32.In a similar manner, a typical ground terminal on the die 10 isconnected by a wire-bonded wire 36 directly to the ground plane 28. Thelarge, low-inductance power-connection planes provided by the conductivelayers 28, 32 can allow additional leads on the lead frame 16 to beavailable forother connections, such as I/O connections. Bonding wires40 are connected between I/O signal bonding pads formed on theintegrated-circuit die 10 and the bonding fingers 20 of the lead frame16. The bonding fingers 20 are exposed on the surface of the substrate14 between the die 10 and the first layer of insulating material 26. Toprovide external power and ground connections, additional bonding wires44 and 46 are connected between the bonding fingers 20 and therespective ground and power planes 28, 32, as illustrated in the Figure.

As a result of using the first conductive layer 28 as a ground plane andofusing the second conductive layer 32 as a power plane, the inductancefor these connections to the integrated circuit die 10 can be reduced,reducing ground bounce and cross talk between neighboring leads. Thisapproach provides a package having improved electrical performance withthe low-cost of a molded plastic package.

FIG. 4 illustrates an upper mold half 50 and a lower mold half 52containing a thermally-enhanced, low inductance intermediate assembly,such as shown in FIG. 3. The assembly has the conductive layers 28, 32FIGS. 2 and 3 formed on the top side of the integrated-circuit die. Thestructure formed by these layers tends to modulate the flow of plasticmolding material in the upper half of the mold. This balances the flowin the lower half of the mold. The result is to reduce formation ofvoids in the molded plastic body of the package, particularly in thelower half of the mold.

After all of the connections are made between the integrated circuit die10, the bonding fingers 20, and the conductive planes 28 and 32 of FIGS.2and 3, the entire assembly is enclosed in a mold cavity formed betweenan upper mold half 50 and a lower mold half 52. The mold cavity is thenfilled with a plastic molding material which encapsulates the assemblyin a molded plastic package body. The plastic molding material is, forexample, a standard molding compound such as provided by the SumitomoCompany as 6300 HS or HG molding compound, or as 7320 C low viscositymolding compound. The arrows show the flow of plastic molding materialthrough the upper mold half 50 and through the lower mold half 52.

The plastic molding compound is injected into an inlet 54 in one side ofthe mold cavity. The molding material then flows both over and under theintegrated-circuit die 10 and the substrate 14 towards the other side ofthe mold cavity where an air vent 56 allows the ambient air originallypresent in the mold cavity to escape. As described above, the conductivelayers 28, 32 of FIGS. 2 and 3 tend to restrict the flow of the moldingcompound on the top side of the assembly such that the flow is balanced.Since there is a balanced flow of plastic material both over and undertheintegrated-circuit die assembly, the flows meet at the air vent 56 onthe side of the mold cavity. This eliminates the formation of voids, orblowholes, in the molded body formed within the mold halves. Thus, theconductive layers not only provide for low-inductance power planes, butalso assist, during the molding process, in reducing voids in the moldedplastic body of the package. The conductive planes formed thereby canoptionally be used for signals as well as for power connections, asrequired for particular applications. Additional conductive layers areformed in a manner similar to the first and second conductive layers,thatis, by spacing additional insulating layers between adjacentconductive layers.

FIG. 5 shows an alternative embodiment of an intermediateintegrated-circuit assembly 60, which uses a conductive slug 62, made ofcopper or silicon carbide to provide enhanced thermal characteristicsand low-inductance connections for an integrated-circuit die. Aninsulating layer 64 is provided, if necessary, to insulate the die 10and the leads from copper slug 62.

Note that while the invention has been described in connection withproviding two conductive layers, a single conductive layer or three ormore layers formed in accordance with the invention provide theadvantagesof the invention.

The foregoing descriptions of specific embodiments of the presentinventionhave been presented for the purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the preciseforms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and itspractical application, tothereby enable others skilled in the art to best utilize the inventionand various embodiments with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

We claim:
 1. An plastic encapsulated electrically and thermally enhancedintegrated circuit comprising:an integrated-circuit die; a thermallyconductive, electrically-insulated substrate having a top surface uponwhich said integrated-circuit die is centrally attached; a lead framehaving inwardly-extending bonding fingers, said lead frame having thebottom side of said bonding fingers attached to the top surface of saidthermally conductive, electrically-insulated substrate, such that saidbonding fingers of said lead frame peripherally surround saidintegrated-circuit die; a first layer of insulating material bonded tothe top side of said bonding fingers of said lead frame, such that saidbonding fingers are disposed between said first contiguous layer ofinsulating material and the top surface of said thermally conductive,electrically-insulated substrate, and such that said first contiguouslayer of insulating material peripherally surrounds saidintegrated-circuit die; a first conductive layer bonded on top of saidinsulating layer such that said insulating layer is disposed betweensaid first conductive layer and said bonding fingers; a second layer ofinsulating material bonded on top of said first contiguous conductivelayer and peripherally surrounding said integrated-circuit die, suchthat said first conductive layer is disposed between said first andsecond insulating layers; a second conductive layer bonded on top ofsaid second contiguous layer of insulating material, such that saidsecond insulating layer is disposed between said first and secondconductive layers; means for electrically connecting saidintegrated-circuit die to said first and second conductive layers; amolded package body formed of a plastic material, which is molded aroundsaid integrated-circuit die, said thermally conductive,electrically-insulated substrate, said insulating layers, saidconductive layers, and said inwardly extending bonding fingers.
 2. Theencapsulated integrated circuit of claim 1 wherein said thermallyconductive, electrically-insulated substrate is formed of aluminanitride material.
 3. The encapsulated integrated circuit of claim 1wherein said thermally conductive, electrically insulated substrate is acopper slug separated from said integrated-circuit die and said bottomside of said leads by a layer of electrically insulating material. 4.The encapsulated integrated circuit of claim 1 wherein said layers ofinsulating material are layers of polyimide.
 5. The encapsulatedintegrated circuit of claim 1 wherein said first and second conductivelayers are used as ground and power planes for said integrated circuit.6. The encapsulated integrated circuit of claim 1 wherein said firstinsulating layer and said first conductive layer have a larger widththan said second insulating layer and said second conductive layer, suchthat the edges of said second insulating and conductive layers are notaligned with the edges of said first insulating and conductive layers toexpose a portion of the surface of said first layer.
 7. An improvedthermally-enhanced package assembly for an integrated-circuit diepackaged in a molded plastic package, comprising:an integrated-circuitdie; a thermally conductive, electrically-insulated substrate having atop surface upon which said integrated-circuit die is attached in adie-up configuration at a central portion of said substrate; a leadframe having inwardly-extending bonding fingers, where the bottomsurfaces of said bonding fingers are attached to said top surface ofsaid thermally conductive, electrically-insulated substrate; aninsulating layer positioned over said bonding fingers of said lead framealong the outer margins of said top surface of said thermallyconductive, electrically-insulated substrate; a conductive layerpositioned over said insulating layer; and one or more bonding wiresconnected between respective bonding pads on said integrated-circuit dieand said conductive layer.
 8. The package assembly of claim 7 includinga molded package body formed of a plastic material, which is moldedaround said integrated-circuit die, said thermally conductive,electrically-insulated substrate, said insulating layer, said conductivelayer, and said inwardly extending bonding fingers.
 9. A plasticencapsulated electrically and thermally enhanced integrated-circuitpackage, comprising:an integrated-circuit die; a thermally conductive,electrically-insulated substrate having a top surface upon which saidintegrated-circuit die is attached in a die-up configuration at acentral region thereof; a lead frame having inwardly-extending leadswith bonding fingers at the inner ends thereof, where the bottomsurfaces of said leads are attached to said top surface of saidthermally conductive, electrically-insulated substrate; a layer ofinsulating material bonded to the top side of said bonding fingers ofsaid lead frame, such that said bonding fingers are disposed betweensaid layer of insulating material and said top surface of said thermallyconductive, electrically-insulated substrate, and such that said layerof insulating material peripherally surrounds said integrated-circuitdie; a conductive layer bonded on top of said layer of insulatingmaterial such that said insulating layer is disposed between saidconductive layer and said bonding fingers; means for electricallyconnecting said integrated-circuit die and said conductive layer; and amolded package body formed of a plastic material, which is molded aroundsaid integrated-circuit die, said thermally conductive,electrically-insulated substrate, said insulating and conductive layers,and said inwardly extending leads.
 10. The encapsulated integratedcircuit of claim 9 wherein said thermally conductive,electrically-insulated substrate is formed of alumina nitride material.11. The encapsulated integrated circuit of claim 9 wherein saidthermally conductive, electrically insulated substrate includes anassembly of a copper slug and a layer of electrically insulatingmaterial, where said copper slug is separated from saidintegrated-circuit die and said bottom side of said bonding fingers bysaid layer of electrically insulating material.
 12. The encapsulatedintegrated circuit of claim 9 wherein said layer of insulating materialis a layer of polyimide.